This disclosure relates generally to standoff detection systems. More particularly, this disclosure may relate to chambers configured to suspend clouds of aerosols for testing and calibration of such systems.
Standoff detection is the remote detection of the presence of a substance. In some cases, the substance to be detected may be aerosolized (i.e. comprise particulates in a cloud). Such remote detection may be advantageous over localized detection, such as through sniffer detectors, in a diverse number of situations, including where the substance is a chemical and/or a biological agent. For example, where the substance is a chemical and/or biological agent used as an aerosolized weapon, remote detection may allow for early warning and response, without those operating the detector already being in a zone of danger. Standoff detection also permits monitoring in areas where localized detectors cannot be feasibly placed, such as over water in a port or on property immediately adjacent to high value strategic assets (e.g. military bases) not under direct ownership of the operating body. Various types of standoff detectors exist, including but not limited to Light Detection and Ranging (LIDAR) standoff detectors. There are currently only a limited number of standoff detectors for chemical and biological threats, notably the Frequency Agile Laser (FAL) and the Joint Biological Standoff Detection System (JBSDS). However, remote sensing of aerosols can be used for other applications using LIDAR and hyperspectral technologies, such as wind shear detection and cloud mapping.
It may be desirable to characterize some standoff detectors based upon their ability to detect signatures of biological or chemical agents at certain concentrations, from a certain distance. This characterization may be useful both for classifying the effectiveness of the detector, and for calibrating the detector. Due to the inherent danger of some biological or chemical agents, simulants may be utilized that approximate the presence of a more hazardous substance, by presenting similar detectable signatures to the detector. A non-exhaustive list of common simulants includes Arizona road dust, powdered egg whites, acetone, polydimethylsiloxanes, and polyethylene glycol.
To generate and isolate an aerosolized cloud of chemical and/or biological agents, or simulants thereof, standoff chambers may be utilized. Depending on the type of detector being tested, different types of chambers suffer from various design inefficiencies. For example, conventional standoff chambers are large structures that are generally immovable. The chambers are constructed in place, necessitating users to bring their detectors to the chamber site in order for them to be tested or calibrated. As an example, the Active Standoff Chamber (ASC), also known as the Vortex Chamber, is located at the Edgewood Chemical Biological Center (ECBC), and measures 20 feet by 24 feet, having a 4 foot large aero-window. One proposed chamber, submitted to the Army's Simulation, Training & Instrumentation Command (STRICOM), was based on the Active Standoff Chamber, but would have been over 100 feet long, utilizing a 10 foot long window. Due to the immense size of prior chamber configurations, value may be found in reducing the size of standoff chambers. For example, a reduced size of chamber may permit the testing of standoff detectors in local laboratory settings, as opposed to large field testing sites. As another example, larger chambers may require a larger amount of simulant or other agent in order to achieve sufficient particulate density for testing purposes. Increased quantities of more hazardous agents present obvious difficulties both from safety and political perspectives. Even some simulants are themselves precursors to chemical or biological weapons, and may be regulated by law or international treaties as to the amount that may be produced (i.e. under nonproliferation agreements). Development of standoff chambers suitable for laboratory use presents a number of challenges, however.
One such challenge in minimizing the size of a standoff chamber is in containing the cloud within the chamber. Some chambers may utilize physical windows, however where the detector being tested is laser based, such windows may necessitate expensive materials such as zinc sulfide or zinc selenide, due to the long wavelengths involved, to minimize scattered returns. While some prior chambers have utilized thin film windows or large tubular constructions, these designs may yield unacceptable scatter or contamination in laboratory settings, or be of an impractical size for a conventional laboratory. For example, the ECBC ASC utilizes a cylindrical geometry, and accordingly its footprint on the ground grows with the square of the path length distance. It is highly advantageous to have a long path-length to more accurately simulate a cloud of aerosol, but the sideways growth of the chamber rapidly eats up valuable laboratory real estate without directly contributing to the testable length.
For certain types of detectors, it may desirable or necessary during calibration or testing for the detector to have an unobstructed view of the cloud (i.e. where observation is not through any transparent shield material), so as to further reduce or eliminate unwanted scatter. One such mechanism to permit unobstructed observation is through the use of aero-windows, which are apertures in the standoff chamber surrounded by a filtering flow of air that is configured to absorb and filter any of the cloud that may exit through the aperture. Such aero-windows present an unobstructed view of the cloud in the chamber, while preventing the cloud from escaping into the surrounding environment. Even though simulants are generally benign, their unrestrained spread is undesirable. In an enclosed laboratory environment, foreign objects and debris is generally strictly controlled, as the spread of even benign particulates through the escape of the cloud may interfere with other experiments.
Another desirable quality in the construction of any size of standoff chamber is the chamber's ability to hold aerosolized particulates in the cloud for long periods of time. In some chambers, loss of suspension of particles in the cloud may result, for example, by the particles entering a low pressure area of the chamber. In some conventional standoff chambers, the chamber can only suspend an aerosol a matter of seconds before the particles drop out, requiring numerous repetitions of tests to obtain sufficient data, adding to test expenses. Chambers such as the ASC improved greatly on prior chamber designs, having the ability to suspend particles on the order of minutes. However, in the ASC, which utilized a horizontal cylindrical airflow to create the cloud, a low velocity zone at the center of the horizontal vortex was created, resulting in an area where particles could fall out of suspension and settle to the chamber floor. This phenomenon affects larger particles more strongly than smaller particles. Such a design, in conjunction with the inherent transfer rate of the particles into the aero-windows, limited the suspension decay time constant of larger particles entrained in the cloud to approximately a minute. Smaller particles, being less affected, would show a concentration decay time constant of approximately five minutes.
What is needed is, among other things, improvements over known standoff chambers, increasing the amount of time that particles can be maintained in a cloud, while being of a design that permits miniaturization for use in a conventional laboratory environment while maintaining a suitable test path length.